Using differential display of messenger RNA, we analyzed acute changes in gene expression after treatment of target cells with IGF-I, a major regulator of cell proliferation/differentiation and somatic growth. Methods: Human B- and T-lymphoblasts were treated for 4 h with a physiological concentration (10 ng/mL) of IGF-I or dilute acetic acid control, either in the presence or absence of 10% fetal bovine serum. Total RNA was DNase-treated, reverse transcribed using “anchored” oligo-(dT)9 primers(30mer total length), and then PCR-amplified in duplicate, using(α-33P)dATP and a second primer which binds at arbitrary but reproducible locations on the cDNA. Following PAGE and autoradiography, cDNA fragments showing putative differential expression were eluted and reamplified for further analysis, including sequencing and use as probes of Northern blots prepared from IGF-I-treated lymphoblasts or for ribonuclease protection assays. Results/Discussion: In addition to interesting candidates such as ubiquitin and cDNAs which do not match known gene sequences, we identified a key component of the respiratory chain leading to cellular ATP synthesis (mitochondrial NADH dehydrogenase subunit 5, ND5) whose mRNA level was up-regulated after 4 h of IGF-I treatment in either B- or T-lymphoblasts, whether serum was concurrently present or not. We are currently characterizing the time course and dose-response characteristics of this change in mitochondrial gene expression after IGF-I treatment. Our findings may have important implications for IGF-I control of cellular energy metabolism and signal transduction to mitochondria, and may lead to new insights into how IGF-I promotes cell proliferation.